This invention relates to the purification of a carbonyl-containing 1,3-propanediol composition. In a specific aspect, the invention relates to treatment of a 1,3-propanediol-containing stream from the manufacture of polytrimethylene terephthalate so as to permit recycle of the 1,3-propanediol to the polymerization process.
Polytrimethylene terephthalate is a polyester useful in making fibers for carpets and textiles. In the preparation of polytrimethylene terephthalate by the esterification/condensation polymerization of 1,3-propanediol and terephthalic acid, the 1,3-propanediol is initially present in the esterification reaction mixture in a molar excess with respect to the acid. In order to achieve high polymer molecular weight, excess 1,3-propanediol is removed under vacuum and condensed as the polycondensation reaction proceeds. This condensed 1,3-propanediol stream typically also contains carbonyl-containing by-products of the polymerization process.
Although it would be desirable to recycle this excess 1,3-propanediol to the polymerization process, the addition of the by-products along with the 1,3-propanediol could result in the accumulation of carbonyl-containing byproducts in the system and interfere with the production of high-quality polyester.
It is known to purify such byproduct streams by adding a substantial amount of water, acidifying the solution, and then distilling the 1,3-propanediol under basic conditions. It would be desirable to recover purified 1,3-propanediol from the byproduct stream without the necessity of adding large quantities of water to the system.
It is therefore an object of the invention to provide a process for preparing polytrimethylene terephthalate in which relatively pure 1,3-propanediol is recovered from the distillate stream and recycled to the polymerization process.